This invention relates to thermoplastic polymer films. In particular, this invention relates to embossed oriented thermoplastic films and to the process for forming these films.
The use of oriented films is widespread, particularly with films comprised of semi-crystalline thermoplastic polymers. These oriented films are characterized by high tensile strength and low-to-moderate elongation. Orientation can also influence crystalline order and hence the melting or softening point of an oriented polymer.
It is known that unoriented polymeric materials, such as polypropylene, can be embossed during casting. However, in the manufacture of oriented polymer films, the cast-embossing process cannot be used. Orientation of a film is carried out by casting a polymer sheet and subsequently heating and stretching (also known as drawing) the sheet into an oriented film. An embossed pattern produced during the casting of the sheet is destroyed by an orientation process.
Embossed polymer films are desirable because of the decorative or other surface effects produced on the films. For example, low gloss values are desirable on embossed polymer films because they are useful, for example, as adhesive-tape backings. Embossing of polymer films is also useful in changing the frictional characteristics of the film (e.g., to alter slip properties) or to alter its adhesion to a surface.
Methods to emboss oriented films have been proposed but these require heating to temperatures and for times that cause the oriented polymer film to lose desirable bulk mechanical properties (such as low-to-moderate elongation and high tensile strength). In addition, such methods tend to destroy desirable surface properties of the films. Therefore, the methods often include an additional surface treatment step after embossing to restore some of the surface properties of the oriented film.
Briefly, in one aspect of the present inventions, an embossed oriented thermoplastic film is provided comprising an oriented thermoplastic film having first and second major surfaces and a plurality of embossments on at least one of the first and second major surfaces wherein the tensile strength-at-break and tensile elongation-at-break of the embossed oriented thermoplastic film are substantially the same as the tensile strength-at-break and tensile elongation-at-break of the unembossed oriented thermoplastic film.
In another aspect, this invention is the method of making an embossed oriented thermoplastic film comprising the steps of providing an oriented thermoplastic film having first and second major surfaces, softening at least one of the first and second major surfaces to produce a softened surface, embossing the softened surface to produce an embossed oriented thermoplastic film, and cooling the embossed oriented thermoplastic film. In a preferred embodiment the combined steps of softening, embossing, and cooling occur in less than 1 second.
In a preferred embodiment, a heat flux is provided by a flame to at least one major surface of an oriented thermoplastic film resulting in a softened surface. The film then is passed between a nip having at least one embossing surface to form embossments on the film. This embossed surface is then cooled to fix the structure of the embossments. The time required to heat, emboss, and cool the embossed oriented thermoplastic film ranges from about 0.05 to about 1 second.
The resultant embossed oriented film has substantially the same bulk mechanical properties of the oriented film before embossing. This method imparts variable topography to an oriented film that leads to variation in such characteristics as gloss, friction, and mechanical adhesion. Further, when the softening step is carried out by flame treatment, this method imparts additional desirable surface characteristics such as improved wetting and chemical adhesion to subsequent coatings. Thus, in a method of this invention, the film is surface treated and embossed in one high-speed operation.
In yet another aspect, this invention is a layered construction comprising an oriented thermoplastic film having first and second major surfaces and a plurality of embossments on at least one of the first and second major surfaces, wherein the tensile strength-at-break and tensile elongation-at-break of the embossed oriented thermoplastic film are substantially the same as the tensile strength-at-break and tensile elongation-at-break of the oriented thermoplastic film before embossing.
In yet another aspect, this invention is a method of making a thermoplastic article by providing a layered construction comprising an oriented thermoplastic film having a major surface, softening the major surface to produce a softened surface, embossing the softened surface to produce an embossed layered construction, and cooling the embossed layered construction, wherein the tensile strength-at-break and tensile elongation-at-break of the oriented thermoplastic film in the embossed layered construction are substantially the same as the tensile strength-at-break and tensile elongation-at-break of the oriented thermoplastic film in the layered construction before embossing.
In this application, xe2x80x9ctensile strength-at-break and tensile elongation-at-break of the embossed oriented thermoplastic film are substantially the same as the tensile strength-at-break and tensile elongation-at-break of the unembossed oriented thermoplastic filmxe2x80x9d means that the tensile strength-at-break of the embossed oriented thermoplastic film has a variance of less than 10 percent from the tensile strength-at-break of the unembossed oriented thermoplastic film and the tensile elongation-at-break of the embossed oriented thermoplastic film has a variance of less than 10 percent from the tensile elongation-at-break of the unembossed oriented thermoplastic film. Preferably the tensile strength-at-break has a variance of less than 5 percent and/or the tensile elongation-at-break has a variance of less than 5 percent. More preferably there is essentially no change in these properties after embossing.